The invention relates to branched aromatic polycarbonates having defined ratios of polymer chain branching points and of defective structures within the polymer chain with respect to one another, and also to a process for producing these branched aromatic polycarbonates. The invention particularly relates to those branched aromatic polycarbonates which by virtue of their production process via transesterification of bisphenols with diaryl carbonates in the melt comprise not only the intended polymer chain branching points, e.g. via trifunctional branching-agent molecules, but also defective structures within the polymer chain in the form of undesired by-products. These defective structures have a disadvantageous effect on the rheology of the resultant polycarbonates during processing thereof in the melt.
The advantageous properties of polycarbonates having controlled branching, in comparison with linear polycarbonates, are particularly utilized during the thermoplastic processing of the said materials. Polycarbonate (PC) is processed inter alia by the extrusion process and the injection-moulding process. In the case of the extrusion process, the shear rates arising are in the range≦1000 [l/s]; high viscosity of the polymer melt is required here for good processability of polycarbonate melts to give extruded items. When branched polycarbonates are being developed in particular for use in extrusion applications there is therefore a need for polycarbonates with adequately high melt viscosity at low shear rates, and this therefore means polycarbonates with pronounced pseudoplasticity.
Branched polycarbonate can be produced by various processes. The first type of polycarbonate to achieve industrial significance was SoIPC, produced by the solution-polymerization process. In the SoIPC process, units of relatively high functionality, and in this case especially trifunctional units, are added in order to provide branching in the PC, examples being 1,1,1-tris(4-hydroxyphenyl)ethane (THPE), isatinbiscresol (IBC), and trimellitic acid, etc.
The second process used in industry is the melt-polycarbonate (MeltPC) process. Polycarbonate which is produced in the melt from organic carbonates, e.g. diaryl carbonates, and from bisphenols, without use of additional solvents, by what is known as the melt-transesterification process, also known as the melt process, is achieving increasing economic importance and is therefore a suitable material for many application sectors.
The production of aromatic polycarbonates by the melt-transesterification process is known and is described by way of example in “Schnell”, Chemistry and Physics of Polycarbonates, Polymer Reviews, Vol. 9, Interscience Publishers, New York, London, Sydney 1964, in D. C. Prevorsek, B. T. Debona and Y. Kersten, Corporate Research Center, Allied Chemical Corporation, Moristown, N.J. 07960, “Synthesis of Poly(ester)carbonate Copolymers” in Journal of Polymer Science, Polymer Chemistry Edition, Vol. 19, 75-90 (1980), in D. Freitag, U. Grigo, P. R. Müller, N. Nouvertne, BAYER AG, “Polycarbonates” in Encyclopedia of Polymer Science and Engineering, Vol. 11, Second Edition, 1988, pages 648-718 and finally in Des. U. Grigo, K. Kircher and P. R. Müller “Polycarbonate” [Polycarbonates] in Becker/Braun, Kunststoff-Handbuch [Plastics handbook], Volume 3/1, Polycarbonate, Polyacetale, Polyester, Celluloseester [Polycarbonates, polyacetals, polyesters and cellulose esters], Carl Hanser Verlag Munich, Vienna 1992, pages 117-299.
It is known that polycarbonates produced by the MeltPC process have defective structures within the polymer chain. The nature and amount of the said defective structures depends on various process parameters, e.g. temperature, residence time, and also especially the nature and amount of the catalyst used. It is moreover known that alkali-metal compounds and alkaline-earth-metal compounds favour the formation of defective structure (see, for example, EP 1369446 B1 and EP 1500671 A1).
The defective structures have inter alia xanthone structures, which are responsible for lowering the melt viscosity at low shear gradients. Defective structures having xanthone structures are therefore particularly undesirable in MeltPC.
There was therefore a need for melt polycarbonates with the advantageous properties of controlled polymer-chain branching points created by using polyfunctional monomers in the synthesis of the polymer, but without the disadvantageous rheological property changes due to the undesired defective structures within the polymer chain.
There was therefore a requirement to produce MeltPC with controlled branching generated by polyfunctional monomers, preferably by trifunctional phenolic compounds and particularly preferably by THPE, during the synthesis process, where the MeltPC has pronounced pseudoplasticity at low shear rates of, for example, ≦1000 [l/s], and at the same time has minimum amounts of xanthone structures within the polymer chain. The ratio of controlled branching points using trifunctional phenols to the entirety of undesired xanthone structures within the polymer chain here should be markedly greater than 8, preferably greater than 15.
Branched melt polycarbonates and production of these with use of trifunctional aromatic hydroxy compounds are in principle known. By way of example, U.S. Pat. No. 5,597,887(A) describes the use of very large amounts, 2 mol % and more, of THPE as branching agent for producing melt polycarbonate, which is then blended in a second step with unbranched PC and equilibrated in the melt, in order to obtain a material that can be blow-moulded. The patent says nothing about the content of, or the avoidance, of xanthone structures within the polymer chain.
Other patents that describe branching points in MeltPC using THPE as branching agent are JP-04-089824, JP-04-175368, JP-06-298925 and EP1472302A1, but nothing is said there about the ratio of branching points to xanthone structures within the polymer chain.
It was therefore an object of the invention to find a simple melt-transesterification process which has no additional steps and which can produce suitably branched polycarbonates and which overcomes the disadvantages of the processes cited above and which can efficiently adjust the abovementioned ratio of branching points to xanthone structures within the polymer chain in the MeltPC.
Surprisingly, it has been found that the use of specifically purified branching agents, preferably of trihydroxyaryl compounds and in particular of 1,1,1-tris(4-hydroxyphenyl)ethane (THPE) for producing branched polycarbonates in the melt-transesterification process produces a branched MeltPC which comprises markedly fewer xanthone structures within the polymer chain than melt polycarbonates produced by using an unpurified commercially available branching agent in the melt-transesterification process. This method can produce melt polycarbonates where the ratio of branching-agent structures to xanthone structures within the polymer chain is markedly greater than 8, preferably greater than 15. The specific purification of the branching agent is undertaken in solution on cation exchangers.